1. Field of the Invention
The present invention relates to a method for real-time estimation of combustion parameters of an internal-combustion engine from the vibratory response of the engine delivered by a vibration detector (accelerometer).
2. Description of the Prior Art
Standards for emission reduction have led to a progressive adaptation of engine technologies such as the introduction of additional actuators (exhaust gas recirculation valves, direct electronic injection and variable-geometry turbines) and new combustion modes (homogeneous diesel combustion or stratified gasoline combustion). In this context, the engine has become an assembly of complex systems requiring management by means of modern automation techniques. The engine control function manages the available actuators to guarantee engine performance while meeting environmental constraints. A fundamental aspect of engine control lies in the combustion control system via precise management of the thermodynamic conditions in the cylinders (temperature, boost pressure, composition of the mixture admitted) and of the adjustment of the injection parameters.
The new combustion modes, such as homogeneous diesel combustion, are much less polluting than conventional combustions (gasoline or diesel), but they are, on the other hand, much more sensitive. They therefore involve an additional investment in terms of regulation. Loop combustion control is possible only if information is available on the combustion progress and if it can be influenced through the control of the available actuators. These parameters can be obtained from direct measurement of the pressure in the combustion chamber by means of a specific detector. This method allows direct access to physical combustion parameters but, because of the life and cost of this type of detectors, they cannot yet be standard in vehicles. To date, it is more realistic to turn towards alternative techniques such as the estimation of combustion parameters from non-intrusive measurements. A synthesis of the existing methods is presented below:
Ionization current interpretation: The principle measures the ionization current generated during combustion. During combustion, some chemical reactions cause release of ions. Their production is influenced by the temperature and pressure conditions in the cylinder. To detect them, the plug is permanently supplied with a low-amplitude voltage. The ionization current contains information on the successive combustion stages: ignition, flame front propagation and a stage associated with the pressure and temperature variations at the end of the reaction. Eriksson and Nielsen have shown in the document mentioned below, for example, that the maximum pressure during a cycle and its argument can be identified by interpreting the ionization current:    L. Eriksson, Spark Advance Modeling and Control, Ph.D. thesis, Linköping University, Sweden, 1999.
Analysis of the instantaneous rotating speed of the crankshaft: The speed is measured by means of an encoder mounted at one end of the crankshaft. The cylinder pressure is reconstructed by inversion of the kinematic chain of the engine, the model thereof being of course known. The following document can be referred to for example:    S. J. Citron, J. E. O'Higgins & L. Y. Chen, Cylinder by Cylinder Engine Pressure and Pressure Torque Waveform Determination Utilizing Speed Fluctuations, SAE Paper (1989), No. 890486.
Reconstruction from engine vibration measurements: The vibrations of an internal-combustion engine can be measured by means of a detector, referred to as accelerometer, mounted on the engine. An accelerometer is a detector that, in a given reference system, measures the acceleration of the body to which it is fastened (or one of its components) and allows the resulting impacts and vibrations to be studied. Extraction of coherent combustion parameters from an accelerometer is difficult because of the nature of the signal that is delivered. In fact, this signal does not only contain information on combustion because the vibratory response of the engine is also due to other events, such as injection nozzle chattering, piston return stroke to the top dead center, vibrations induced by the distribution system, etc.
The use of accelerometer type detectors has already been described in previous books. The following documents, wherein the cylinder pressure is to be estimated from the signal delivered by the accelerometer, are exemplary:    Y. Gao & R. B. Randall, Reconstruction of Diesel Engine Cylinder Pressure Using a Time Domain Smoothing Technique, Mechanical Systems and Signal Processing 13 (1999), no. 5, 709-722,    H. Du, L. Zhang & X. Shi, Reconstructing Cylinder Pressure from Vibration Signals Based on Radial Basis Function Networks, Proceedings of the Institution of Mechanical Engineers, Part D 215 (2001), 761-767,    R. Johnsson, Cylinder Pressure Reconstruction Based on Complex Radial Basis Function from Vibration and Speed Signals. Mechanical Systems and Signal Processing, 2006.
Du et al. address with the cylinder pressure estimation problem by means of signal processing techniques involving deconvolution or inverse filtering. They have shown the capacity of a neural network to carry out an approximation of the non-linear transfer function between the evolution of the cylinder pressure and the generated vibratory signal. Johnsson uses a similar method according to which the signal delivered by the accelerometer is combined with the engine speed signal to reconstruct the pressure signal and the combustion parameters by means of a neural network whose inputs are imaginary variables.
Other methods using accelerometric detectors for controlling the combustion of an engine are also known:
EP Patent 1,116,946 describes a method and a system for controlling the combustion from signals delivered by an accelerometer, according to which the measured signal is windowed (combustion occurrence period). This signal is then processed during each control loop and it is compared with a reference signal in order to determine the changes to be made to the combustion parameters. Processing the signal from the accelerometer comprises three main stages: rectification, filtering and integration.
U.S. Pat. No. 2,004,267,430 (WO-05,001,263) describes a method of processing accelerometric signals resulting from the vibrations of an internal-combustion engine. In particular, the signals are filtered by spectral filters and the combustion analysis curve is reconstructed by deconvolution of a transfer function identified from an experimental database. The result is an estimation of the combustion parameters allowing combustion control. For example, the parameter referred to as SoC (Start of Combustion) is estimated from a polynomial depending on the energy release, its maximum angle and the boost pressure.
German Patent 19,536,110 (French Patent 2,739,414) describes a method of processing accelerometric signals resulting from vibrations for controlling the combustion of a diesel engine. In particular, the signals are filtered in two different frequency bands. The first frequency band [10 kHz, 30 kHz] allows extraction of components associated with the injection by means of a thresholding device. The second frequency band [0.5 kHz, 4 kHz] allows extraction of the signal components generated by the combustion using an identical thresholding method.
French Patent 2,834,789 describes a knock signal processing method. The method adjusts the acquisition sampling frequency to the engine speed. The Fourier transform is calculated for the various sampling value groups for a frequency of interest. The information on the combustion results from the summation of the Fourier transform results.
The parameters from the aforementioned approaches cannot be used directly for applications linked with internal-combustion engine diagnosis and control. These methods are based on a temporal integration of a signal delivered by an accelerometer. Processing of the signal from the accelerometer is not carried out in real time. Furthermore, these methods greatly depend on the nature of the combustion and/or on the technology of the detector used. Their field of application is therefore limited.
The present invention provides a method for controlling the combustion of an internal-combustion engine, regardless of the nature of the combustion and of the technology of the vibration detector that is used, from processing of the signal delivered by an accelerometer based on a real-time estimation of the harmonic decomposition of the accelerometric signal. The method provides real-time calculation of combustion state indicators.